1. Field of the Invention
The present invention relates, in general, to an apparatus and method for controlling motors and, more particularly, to an apparatus and method for controlling a brushless direct current motor, which minimizes torque ripple.
2. Description of the Related Art
As is well known to those skilled in the art, a brushless Direct Current (DC) motor employs a rectifying circuit including switching devices instead of mechanical elements, such as a brush and a commutator. The brushless DC motor is characterized in that the replacement of the brush due to abrasion is not necessary, and little electromagnetic interference and drive noise exist. A control apparatus driving the brushless DC motor is described in detail with reference to FIG. 1.
FIG. 1 is a block diagram of a conventional apparatus to control a brushless DC motor. As shown in FIG. 1, a power converting device including a converter 104, a capacitor 108 and an inverter 106 converts Alternating Current (AC) power supplied from an AC power supply unit 102 to pulse-shaped 3-phase AC power, and supplies the 3-phase AC power to a brushless DC motor (BLDC motor) 110. Of U, V and W phase currents of the 3-phase AC power supplied to the brushless DC motor 110 from the inverter 106, U and V phase currents are detected by first and second current sensors 112a and 112b. The U and V phase current information detected by the first and second current sensors 112a and 112b, respectively, is provided to a control unit 114 and inverter control signals provided to the inverter 106 are based on the detected U and V phase current information. A position and a speed of a rotator of the brushless DC motor 110 are detected by a position/speed detecting unit 116. The detected position/speed information of the rotator is further provided to the control unit 114 and inverter control signals provided to the inverter 106 are based on the detected position/speed information. That is, the control unit 114 controls the speed of rotation of the brushless DC motor 110 with reference to the phase current information inputted from the current sensors 112a and 112b and the rotator position/speed information inputted from the position/speed detecting unit 116. The control unit 114 outputs inverter control signals to control phase commutation times of the 3-phase AC power outputted from the inverter 106 and the intensities of phase currents thereof, thus allowing the rotation speed of the brushless DC motor 110 to comply with a speed command.
Torque of the brushless DC motor may be expressed by the multiplication of an induced voltage by a current. In phase commutation periods of the 3-phase AC power, phase currents are temporarily decreased, thus generating torque ripple. Since the torque ripple is a cause of noise generation and vibration, a plan to minimize the torque ripple is required.
To minimize the torque ripple of the brushless DC motor, phase currents temporarily reduced during the phase commutation periods must be provided with compensation. The compensation must be performed only during the phase commutation periods. If the phase commutation periods are not accurately detected, undercompensation or overcompensation of the phase currents occurs. Accordingly, to minimize the torque ripple, accurately detecting the phase commutation periods is important. In the related art, current sensors, such as hall effect sensors, are used to detect the phase commutation periods of the brushless DC motor. However, using the current sensors to detect respective phase currents, as shown in FIG. 1, increases a manufacturing cost of a brushless DC motor control apparatus, thereby imposing a heavy burden to both a manufacturer and a user.